CSE 451: Operating Systems Winter 2012 Secondary Storage

1. CSE 451: Operating SystemsWinter 2012Secondary Storage Mark Zbikowski Gary Kimura

2. Secondary storage • Secondary storage typically: • is anything that is outside of “primary memory” • does not permit direct execution of instructions or data retrieval via machine load/store instructions • Characteristics: • it’s large: 50-1000GB (or more!) • it’s cheap: $0.25/GB … er… $0.10/Gb • it’s persistent: data survives power loss • it’s slow: milliseconds to access • why is this slow? • it does fail, if rarely • Big failures (disk dies) • Little failures (read/write errors, one byte in 10^13)

3. Another trip down memory lane … IBM 2314 About the size of 6 refrigerators 8 x 29MB (M!) Required similar sized air conditioning

4. Disk trends • Disk capacity, 1975-1989 • doubled every 3+ years • 25% improvement each year • factor of 10 every decade • Still exponential, but far less rapid than processor performance • Disk capacity since 1990 • doubling every 12 months • 100% improvement each year • factor of 1000 every decade • Capacity growth10x as fast as processor performance! • Speed has NOT grown similarly

5. Only a few years ago, we purchased disks by the megabyte (and it hurt!) • Today, 1 GB (a billion bytes) costs $1 $0.50$0.25 $0.10 from Dell (except you have to buy in increments of 40 80250 500 GB) • => 1 TB costs $1K $500 $250 $100, 1 PB costs $1M $500K $250K $10000 • Technology is amazing • Flying 747 six inches above the ground at 600mph • Reading/writing a strip of postage stamps • 5-20 molecules of gas separating platter from head • But… • Jet bumps down • Bits are so close that cosmic rays/quantum effects change them

6. Memory hierarchy • Each level acts as a cache of lower levels 100 bytes CPU registers ~ .1 ns 32KB L1 cache 1 ns 256KB L2 cache 4 ns 1GB Primary Memory 60 ns 100GB Secondary Storage 10+ ms Tertiary Storage 1s-1hr 1-1000TB

7. Memory hierarchy: distance analogy 1 second CPU registers “My head” 12 seconds L1 cache “My desk” 1 minutes L2 cache “This room” 15 minutes Primary Memory “This building” 4.75 years Secondary Storage “This city” Tertiary Storage 500 years “This planet”

8. Big Picture • OS provides abstractions to allow physical HW resources to be shared / protected • CPU sharing with threads (virtual CPU) • Memory sharing with virtual memory • Disk sharing with files

9. Disks and the OS • Disks are messy, messy devices • errors, bad blocks, missed seeks, etc. • Job of OS is to hide this mess from higher-level software • low-level device drivers (initiate a disk read, etc.) • higher-level abstractions (files, databases, etc.) • OS may provide different levels of disk access to different clients • physical disk block (head, cylinder, sector) • disk logical block (disk block #) • file logical (filename, block or record or byte #)

10. Physical disk structure • Disk components • platters • surfaces • tracks • sectors • cylinders • arm • heads sector track surface cylinder platter arm head

11. Disk performance • Performance depends on a number of steps • seek: moving the disk arm to the correct cylinder • depends on how fast disk arm can move • seek times aren’t diminishing very quickly (why?) • rotation (latency): waiting for the sector to rotate under head • depends on rotation rate of disk • rates are increasing, but slowly (why?) • transfer: transferring data from surface into disk controller, and from there sending it back to host • depends on density of bytes on disk • increasing, relatively quickly • When the OS uses the disk, it tries to minimize the cost of all of these steps • particularly seeks and rotation

12. Performance via disk layout • OS may increase file block size in order to reduce seeking • OS may seek to co-locate “related” items in order to reduce seeking • blocks of the same file • data and metadata for a file

13. Performance via caching, pre-fetching • Keep data or metadata in memory to reduce physical disk access • problem? • If file access is sequential, fetch blocks into memory before requested

14. Performance via disk scheduling • Seeks are very expensive, so the OS attempts to schedule disk requests that are queued waiting for the disk • FCFS (do nothing) • reasonable when load is low • long waiting time for long request queues • SSTF (shortest seek time first) • minimize arm movement (seek time), maximize request rate • unfairly favors middle blocks • SCAN (elevator algorithm) • service requests in one direction until done, then reverse • skews wait times non-uniformly (why?) • C-SCAN • like scan, but only go in one direction (typewriter) • uniform wait times

15. Interacting with disks • In the old days… • OS would have to specify cylinder #, sector #, surface #, transfer size • i.e., OS needs to know all of the disk parameters • Modern disks are even more complicated • not all sectors are the same size, sectors are remapped, … • disk provides a higher-level interface, e.g., SCSI • exports data as a logical array of blocks [0 … N] • maps logical blocks to cylinder/surface/sector • OS only needs to name logical block #, disk maps this to cylinder/surface/sector • on-board cache • as a result, physical parameters are hidden from OS • both good and bad

16. Example disk characteristics